Page 31 - Advanced Gas Turbine Cycles
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8 Advanced gas turbine cycles
Tt
0' I I t
S
Fig. 1.8. Temperature-entropy diagram for a Carnot cycle (after Ref. [l]).
It has three features which give it maximum thermal efficiency:
(i) all processes involved are reversible;
(ii) all heat is supplied at the maximum (specified) temperature (T-);
(iii) all heat is rejected at the lowest (specified) temperature (Tmin).
In his search for high efficiency, the designer of a gas turbine power plant will attempt
to emulate these features of the Carnot cycle.
1.4. Limitations of other cycles
Conventional gas turbine cycles do not achieve Carnot efficiency because they do not
match these features, and there exist
(i) 'external irreversibilities' with the actual (variable) temperature of heat supply being
less than T,, and the actual (variable) temperature of heat rejection being greater
than Tmin;
(ii) 'internal irreversibilities' within the cycle.
Following Caput0 [4], we define mean temperatures of heat supply and rejection as
T= QB T= QA (1.11)
B - -9 A- $!g
~QB
I,
Parameters & and tA are then defined to measure the failures to achieve the maximum and
minimum temperatures T,, and Tmin,
(1.12)
where & is less than unity and tA is greater than unity. The combined parameter
(1.13)
